The need for reconstructive surgery of teeth by means of full or partial upper and/or lower dentures is common, particularly among older people and those who have suffered an injury.
The reconstructive process typically begins by the dentist taking impression molds of the patient's remaining teeth and gums. These impressions are made by having the patient bite into soft impression material held in a U-shaped tray that fits around the teeth and gums. These negative molds, when hardened, are then used to make models by casting, in wax and/or plaster or similar plastic materials, of the tooth and gum structure. These positive models are then mounted in a device called an articulator, by a technician who then makes preliminary adjustments for bite interface, fit and appearance. The technician sends the models to the dentist, who proceeds to make further adjustments while fitting the models to the patient. the adjusted models are then used as the basis for the final prothesis.
A major drawback in the foregoing procedure is relating the initial models, taken by the dentist, to positions of the models in the articulator, so the technician can adjust the models for the correct mouth fit. It should be noted that even if the prosthesis fits well over the gums, improper mouth fit, caused by either improper tooth position or an incorrect relationship between the upper and lower teeth is a major source of patient discomfort. Rework is costly, time consuming and often produces even more patient complaints.
Proper, accurate measurement of the occlusal plane will greatly reduce the foregoing problems. It will also simplify the process of checking fit during the intermediate stages when working with the preliminary molds in the patient's mouth, prior to creating the final prosthesis.
Three methods have been most frequently used to reduce the problems mentioned above. The first is by the use of instruments called "face bows." A second method is by the use of instruments that trace the "gothic arch" of lower jaw motion. The third method has been to use no instruments at all, but rather for a dentist to "eyeball" the measurements and trust to luck and a skilled technician at the articulator.
A face bow, as the name implies, is a bow-shaped metallic device whose ends are referenced to some feature of the human skull. Nearly all face bows are oriented horizontally and the ends are referenced to the ear holes. For example, U.S. Pat. Nos. 3,084,438 and 3,200,497 (Goodfriend) relate to horizontal bows which rest in the ears, using plugs, and on the bridge of the nose. U.S. Pat. No. 1,188,416 (Dalbey) pertains to a device for measuring the occlusal plane. The device is a horizontal face bow, connected to the ear holes and resting on the bridge of the nose and under the nose.
A few face bows are referenced to the temporal-mandibular-joint (TMJ), but such referencing is very difficult, and therefor such face bows are rarely employed. For example, U.S. Pat. No. 4,411,622 (Hansen) discloses a device for locating the jaw hinge (TMJ) rather than using the ear holes for a bow reference. Typically, the ends of the horizontal bow have tapered plugs which are inserted into the ears. A third reference point is needed to establish a reference plane to measure from, and the bridge of the nose is often used as such a reference point. Occasionally the third reference point is the forehead or just a level indicator. See for example, U.S. Pat. No. 5,176,515 (Andrews) which illustrates a dental treatment method and apparatus.
The flaw in the foregoing method is that the human face is not symmetrical and ear holes are "soft" mountings with respect to the bone structure. As a result, the bow will usually be tilted and off center. Furthermore, when reset for a later measurement, the new setting will most likely not be in the same plane due to the soft mounting. To summarize, the measurements are not closely repeatable and are not very accurate.
Since the foregoing bows are all entirely mechanical in adjustment and in setting, they are awkward, slow, and often hard to read as the fiducial markings must be poorly placed. The measurements are made by attaching a probe to the bow and then moving it in height and inward toward the teeth. The probe must just touch the edge of the teeth (or tooth) or the gums where there are no existing teeth. Such positioning requires some delicacy and skill and the patient must remain as motionless as possibly for a significant period of time. In addition, horizontal face bows make no measurements with reference to the lower jaw, which leaves out important data for the articulator technician.
Vertical face bows are rare, and the present inventors are only aware of one patent to Sorenson, U.S. Pat. No. 1,976,045. Such a vertical face bow eliminates some of the drawbacks inherent in the use of horizontal bows. In particular, the vertical face bow is centered, it rests against firm cranial features, namely the nasal bridge and chin, and has a lower jaw reference. A vertical face bow is also potentially more repeatable. However, a vertical face bow requires more skill on the part of the dentist to take advantage of its design.
Some instruments establish the bite plane by means of tracing the movement of the teeth horizontally. An example is U.S. Pat. No. 2,656,603 to Brassie which relates to an instrument for relating casts to dental articulators and traces the horizontal movements of the lower jaw to measure aspects of the occlusal plane. A frame is fastened to the head and a flat plate is attached and inserted into the mouth. The lower Jaw is moved in a horizontal fashion. The teeth trace out a pattern resembling a gothic arch on a sheet of paper attached to the plate. While this pattern establishes the entire bite plane, rather than just the front edge, the method suffers from all the drawbacks of a horizontal face bow plus it is more awkward to use and to reference. It is seldom used except for the different function of bite or jaw analysis.
Oddly enough, many dentists take no occlusal plane measurements at all. They rely on the skills of the technician working with the articulator, plus their own skills during multiple fittings. Usually, they arrive at a good result; despite the fact that this takes considerable time of both the dentist, his or her staff, the technician, and not least the patient. Of course, this drives up the cost and exposes all to the risk of error. Major errors, and consequent rework, may necessitate scrapping the initial prosthesis and starting again.
The reason that this method is so often used is that the other methods work so poorly, for the reasons stated previously. The truth is that no measurements, or rather the visual techniques of the dentist and technicians are rarely worse than using the available instruments.
The instruments can produce useful data in the hands of a highly skilled dentist, but this is just the person who can estimate values well without instruments.
What is required is an instrument that is easy to use, quick, accurate, and can take repeatable measurements that are of value to both dentist and technician. The proposed invention is such an instrument.